1. Field of the Invention
The present invention relates to a noise reducing apparatus of a compression/expansion type for preventing deterioration of the signal to noise ratio caused by a noise in a signal transmission system. More specifically, the present invention relates to an improvement in a frequency region split type noise reducing apparatus employing compression/expansion for effectively suppressing a noise breathing phenomenon occurring in a signal transmission system.
2. Description of the Prior Art
A noise reducing apparatus employing signal compression/expansion has been proposed and put into practical use in a signal transmission system such as tape recorders, disc record players and the like for the purpose of preventing deterioration of the signal to noise ratio of the signal because of a narrow dynamic range of a record medium in such signal transmission system. Typical prior art noise reduction systems comprise a Dolby system, a dbx system and the like. The Dolby system is described in detail in, for example, British Pat. No. 1,120,541 and U.S. Pat. No. 3,631,365. On the other hand, the dbx system is described in detail in, for example, U.S. Pat. Nos. 3,681,618; 3,714,462; and 3,789,143.
FIG. 1 shows a block diagram of such a compression/expansion type noise reducing apparatus applied to a tape recorder. In the art, the term "compression/expansion" is referred to as "compansion" and the term "compansion" is often used herein to mean "compression/expansion". The noise reducing apparatus shown in FIG. 1 basically comprises an input terminal 1 adapted to receive an input signal Si, a compressor A including a voltage controlled amplifier 3 connected to receive the input signal and a level sensor 5 for detecting the level of the signal for providing a voltage control signal to the voltage controlled amplifier 3, a tape recorder 11 connected to record the output of the voltage controlled amplifier 3 on a record medium such as a magnetic tape, an expander B including another voltage controlled amplifier 4 connected to receive a reproduced signal from the tape recorder 11 and a level sensor 6 for detecting the level of the signal for providing a voltage control signal to the voltage controlled amplifier 4, and an output terminal 2 adapted to withdraw the output signal So. As well known, the voltage controlled amplifier 3 is structured to compress in a linear manner in terms of a logarithmic scale the dynamic range of the signal as a function of a voltage control signal. On the other hand, the voltage controlled amplifier 4 is structured to expand in a linear manner in terms of a logarithmic scale the dynamic range of the signal as a function of a control signal.
FIG. 2 is a graph showing a compansion characteristic in the case where the compansion coeffecient is 2, wherein the abscissa indicates the input level In and the ordinate indicates the output level Out, the line "a" showing a compression characteristic and the line "b" showing an expansion characteristic.
Thus, according to the noise reducing apparatus shown in FIG. 1, the dynamic range of the signal is compressed and expanded in a linear manner in terms of a logarithmic scale. Referring to FIG. 2, for example, an input signal having a dynamic range of 100 dB is compressed into a signal having a dynamic range of 50 dB and is recorded, while a signal having a dynamic range of 50 dB obtained from the tape recorder 11 is expanded into a signal having the original dynamic range of 100 dB and is withdrawn from the output terminal. Referring to FIG. 1, the compressing operation is achieved by the voltage controlled amplifier 3 serving as a voltage controlled variable gain circuit which is operable to have the gain thereof variable as a function of the signal level output from the level sensor 5. Similarly, the expanding operation is achieved by the voltage controlled amplifier 4 serving as a voltage controlled variable gain circuit which is operable to have the gain thereof variable as a function of the signal level output of the level sensor 6. More specifically, the voltage controlled amplifier 3 is structured to reduce the gain thereof in reverse proportion to an increase of a control signal for the purpose of compressing operation, whereas the voltage controlled amplifier 4 is structured to increase the gain thereof in proportion to the increase of the voltage control signal for the purpose of the expanding operation.
It has been observed that a noise breathing phenomenon occurs in such a compression/expansion type noise reducing apparatus. More specifically, a noise occurring in a signal transmission system is modulated because of variation of the gain of the above described voltage controlled amplifiers and accordingly a noise is heard as level varying. In case of a tape recorder, for example, a hissing noise occurs in case of reproduction of the signal. It has also been observed that a hissing noise is rather similar to a white noise in terms of the spectrum and is very offensive to the ear in the high frequency region, as readily understood from the Noise Rating Number curve. Thus, if such a noise reduction apparatus as shown in FIG. 1 is applied to a tape recorder having the above described noise characteristic, the level of the hissing noise varies over a full range of the frequencies as a function of variation of the level of the recorded audio signal, which makes the reproduced sound offensive to the ear. If and when the recorded audio signal includes frequency components of a broad spectrum to extend even to the high frequency region, then the hissing noise is masked by such frequency components of the audio signal in the high frequency region and the uncomfortableness is substantially mitigated. However, on the average, an ordinary audio signal has an energy distribution which is rather dominant in the lower and middle frequency regions and therefore the hissing noise of the high frequency region is liable to be less masked.
Referring again to FIG. 1, the noise reducing apparatus shown in FIG. 1 employs a preemphasis circuit 7 for emphasizing a high frequency region and a deemphasis circuit 8 having a complementary characteristic for the purpose of eliminating the above described shortcomings in a prior art compansion type noise reducing apparatus. More specifically, according to the FIG. 1 noise reducing apparatus, an audio signal component in the high frequency region where a hissing noise is likely to be conspicuous is in advance emphasized and then recorded, whereupon the signal is processed to return to the original state in reproduction, in order to improve the signal to noise ratio in the high frequency region. However, in order for this process to be effective, it is required that a tape recorder has a broad dynamic range sufficient to accomodate the amount of emphasis in the high frequency region. In actuality, however, the dynamic range in the high frequency region in a typical tape recorder presently available is narrower as compared with the dynamic ranges for middle and lower frequency regions and therefore it is very difficult to have a sufficient amount of range for emphasis in the higher frequency region in such a tape recorder.
Referring again to FIG. 1, the noise reducing apparatus shown in FIG. 1 further employs weighting circuits 9 and 10 coupled to the inputs of the level sensors 5 and 6 for the purpose of supplementing the preemphasis and deemphasis. To that end, the weighting circuits 9 and 10 are structured to exhibit a frequency characteristic wherein a higher frequency region is emphasized. More specifically, when a signal is received wherein energy is dominant in the higher frequency region, the voltage controlled amplifier 3 for the compressing operation is controlled by the weighting circuit 9 to lower the gain thereof, in order to avoid saturation in the higher frequency region by virtue of the emphasis. Nevertheless, another problem is encountered in that the signal to noise ratio throughout the full frequency range is lowered by the weighting operation.
Since the above described emphasis in the higher frequency region raises the energy level of the signal in the higher frequency region, the same serves to reduce relatively the influence on the lower frequency region. As a result, a noise breathing phenomenon in the higher frequency region can be reduced to some extent, although it is still insufficient. It has been observed that such a problem is more aggravated in case of a music sound including piano tones. This is accounted for by the fact that since piano tones have a frequency spectrum of a simple structure similar to a pure tone, the piano tones inherently have a much less masking effect upon a noise and in addition the energy distribution of the piano tones is more dominant in the middle and lower frequency regions, so that the piano tones are much less effective in performing a masking operation upon a varying noise in the higher frequency region. In addition, the improvement by means of emphasis, as described above, is merely aimed to reduce a noise in the higher frequency region which is inoffensive to the ear and is not effective in the reduction of a noise breathing phenomenon in the lower frequency region. It has been observed from the Noise Rating Number characteristic that if the noise level is constant then a noise in the lower frequency region is less conspicuous as compared with a noise in the higher frequency region but if and when the noise level is varying the noise is more likely to be perceived by a listener. Thus, it is in the lower frequency region desired that some expedient is provided for eliminating the above described problems.
As described previously, the reason why a noise breathing phenomenon is less effectively reduced in case of piano tones by means of the FIG. 1 noise reducing apparatus is that although the signal energy distribution is dominant in the middle and lower frequency regions the compression/expansion operation is effected over the full frequency range, which causes a noise in the higher frequency region where no sound signal exists to be heard without any masking effect thereupon.
In order to solve the above described problem, it has been proposed that a full frequency range be split into a plurality of frequency regions and each frequency region be separately subjected to compansion rather than the full frequency range being simultaneously subjected to compansion. More specifically, according to such a frequency region split type noise reducing apparatus, in a frequency region where a sound signal exists, no variation of the noise level is perceived as a result of a masking operation, whereas in a frequency region where no sound signal exists, a noise component is not modulated by a sound signal and is fully suppressed by virtue of the expanding operation, with the result that a noise component is much less perceived.
FIG. 3 shows a block diagram of a frequency region split type noise reducing apparatus employing compansion, wherein a frequency range is split into two frequency regions. The noise reducing apparatus shown in FIG. 3 basically comprises a frequency region split type compressor A connected between an input terminal 1 adapted to receive an input signal Si and a tape recorder 11 and a frequency region split type expander B connected between the tape recorder 11 and an output terminal 2 adapted to withdraw an output signal So. The compressor A comprises a lower frequency compressor including a low pass filter 16, a voltage controlled amplifier 3 connected to receive the output of the low pass filter 16, and a level sensor 5 for detecting the signal level for providing a voltage control signal to the voltage controlled amplifier 3. There is also a high frequency compressor including a high pass filter 18, a voltage controlled amplifier 12 connected to receive the output of the high pass filter 18 and a level sensor for detecting the signal level for providing a voltage control signal to the voltage controlled amplifier 12. An adder 20 is provided for adding the outputs of the voltage controlled amplifiers 3 and 12 for providing a signal being recorded Sr. Similarly, the expander B comprises a lower frequency expander including a low pass filter 17, a voltage controlled amplifier 4 connected to receive the output of the low pass filter 17 and a level sensor 6 for detecting the signal level for providing a voltage control signal to the voltage controlled amplifier 6. There is also a high frequency expander including a high pass filter 19, a voltage controlled amplifier 13 connected to receive the output of the high pass filter 19 and a level sensor 15 for detecting the signal level for providing a voltage control signal to the voltage controlled amplifier 13. An adder 21 is provided for adding the outputs of the voltage controlled amplifiers 4 and 13 for providing the output signal So. As readily understood, the low pass filters 16 and 17 and the high pass filters 18 and 19 are used to split the frequency range into two frequency regions. In determining the frequency regions for these filters, a crossover frequency fc where the frequency regions are overlapped is selected such that a noise breathing phenomenon is effectively minimized in consideration of the foregoing discussion. The voltage controlled amplifiers 3 and 4 and 12 and 13 and the level sensors 5 and 6 and 14 and 15 are structured in a manner similar to the voltage controlled amplifiers 3 and 4 and the level sensors 5 and 6 discussed with reference to FIGS. 1 and 2, except that the voltage controlled amplifiers 3 and 4 and the level sensors 5 and 6 are adapted for compansion in the lower frequency region while the voltage controlled amplifiers 12 and 13 and the level sensors 14 and 15 are adapted for compansion in the higher frequency region. Since the frequency range is split into two frequency regions for the purpose of compression and expansion, the adder 20 is provided to combine the separate signals separately compressed in the split frequency regions and similarly the adder 21 is provided to combine the separate signals separately expanded in the split frequency regions.
In spite of the purpose for separately achieving compansion in the split frequency regions for reducing a noise breathing phenomenon by such a frequency region split type noise reducing apparatus as shown in FIG. 3, another problem arises in that an input signal frequency spectrum is not reproduced with fidelity as a result of compansion. This problem is caused by a frequency characteristic of the low pass filters 16 and 17 and the high pass filters 18 and 19. FIG. 4 shows the frequency characteristics of the low pass filters and the high pass filters, wherein the abscissa indicates the frequency and the ordinate indicates the attenuation, the curve identified as LPF showing the frequency characteristic of the low pass filters and the curve identified as HPF showing the frequency characteristic of the high pass filter. As seen in FIG. 4, a combination of the low pass filters and the high pass filters usually include a crossover region and the crossover region cannot be avoided, however sharp the cut off characteristic of the filters presently available are utilized. Therefore, a signal in the crossover region subjected to a level variation by the voltage controlled amplfier 3 in the lower frequency region on the occasion of compression is also subjected to a level variation by means of the voltage controlled amplifier 13 in the higher frequency region on the occasion of expansion. Conversely, the signal in the crossover region subjected to a level variation by the voltage controlled amplifier 12 in the higher frequency region on the occasion of compression is also subjected to a level variation by the voltage controlled amplifier 4 in the lower frequency region on the occasion of expansion.
Now, let it be assumed that the input signal is Si, the output signal So, the signal being recorded by the tape recorder is Sr, and the frequency characteristics of the low pass filter and the high pass filter are YL(.omega.) and YH(.omega.), respectively. Then .vertline.YL(.omega.)+YH(.omega.).vertline.=1. Further let it be assumed that the gains of the voltage controlled amplifiers 3 and 12 for compression are GL.sup.c and GH.sup.c, respectively, and the gains of the voltage controlled amplifiers 4 and 13 for expansion are GL.sup.e and GH.sup.e, respectively. Then, in order to achieve complementary compression and expansion in recording and reproduction, naturally the following equation must be satisfied. EQU GL.sup.c =1/GL.sup.e .multidot.GH.sup.c =1/GH.sup.e ( 1)
However, in the FIG. 3 noise reducing apparatus, the signals being detected by the level sensors 5 and 14 for compression becomes as follows. EQU GL.sup.c .multidot.YL(.omega.).multidot.Si (2) EQU GH.sup.c .multidot.YH(.omega.).multidot.Si (3)
On the other hand, the signal being recorded Sr is expressed as follows.
Sr={GL.sup.c .multidot.YL(.omega.)+GH.sup.c .multidot.YH(.omega.)}.multidot.Si (4)
Therefore, the signal signals being detected by the level sensors 6 and 15 for expansion are expressed by the following equations. EQU YL(.omega.).multidot.Sr={GL.sup.c .multidot.YL(.omega.)+GH.sup.c .multidot.YH(.omega.)}.multidot.YL(.omega.).multidot.Si (5) EQU YH(.omega.).multidot.Sr={GL.sup.c .multidot.YL(.omega.)+GH.sup.c .multidot.YH(.omega.)}.multidot.YH(.omega.).multidot.Si (6)
Since the signals being detected by the level sensors 5 and 14 for compression and the signals being detected by the level sensors 6 and 15 for expansion are different, it follows that the gains of the voltage controlled amplifiers 3 and 12 and the gains of the voltage controlled amplifiers 4 and 13 are not complementary to each other. The output signal So obtainable at the output terminal 2 is expressed by the following equation. EQU So={GL.sup.c .multidot.GL.sup.e .multidot.YL.sup.2 (.omega.)+GH .sup.c .multidot.GH.sup.e .multidot.YH.sup.2 (.omega.)+(GH.sup.c .multidot.GL.sup.e +GL.sup.c .multidot.GH.sup.e).multidot.YL(.omega.).multidot.YH(.omega.).multidot.YH( .omega.)}.multidot.Si (7)
Referring to the equation (7), the term indicated in the bracket in equation (7), representing the overall transfer characteristic of the FIG. 3 apparatus becomes unity only of and when GL.sup.c =GL.sup.e =GH.sup.c =GH.sup.e =1. In actuality, however, the gains of the voltage controlled amplifiers are variable as a function of the voltage control signal and therefore the condition of GL.sup.c =GL.sup.e =GH.sup.c =GH.sup.e =1 is never met by the FIG. 3 apparatus. In other words, as a matter of fact, the term indicated in the bracket in equation (7) never becomes unity and accordingly Si.noteq.So. Thus, by virtue of a crossover of the signal among the lower and higher frequency regions, the output signal So becomes different from the intput signal Si. The extent of the difference between the input and the output signals Si and So is variable as a function of a level variation in the lower and higher frequency regions. As a result, according to the FIG. 3 apparatus, the fidelity of the signal after expansion is considerably deteriorated.
FIG. 5 is a graph showing qualitatively the manner of deterioration of the fidelity as discussed in the foregoing based on equation (7), wherein the abscissa indicates the frequency and the ordinate indicates the response. A gentle rise in the vicinity of the crossover frequency fc shows the above described deterioration of the fidelity. As described previously, the extent of the rise varies as a function of a variation of the gain of each voltage controlled amplifier by virtue of a variation of an energy distribution of the signal frequency spectrum.
As understood from the foregoing description, although a frequency region split in a noise reducing apparatus employing compansion for separate compansion in split frequency regions is very effective for reduction of a noise breathing phenomenon, another problem arises that the fidelity is considerably degraded, however sharp frequency characteristic filters presently available are employed.